What causes vinegar smell in sweat?

Sweat contains propionic acid, which smells of vinegar. The acid is formed when propionibacteria that live in the ducts of sebaceous glands of adolescent and adult humans break down amino acids.

Although sweat plays a role in the excretion of toxins and waste products from the body, it is virtually odorless to humans when it has not been broken down by bacteria. This is because sweat is mainly composed of water, minerals, lactate and urea. However, its chemical composition and, therefore, smell may vary depending on the sweat glands that it came from as well as an individual’s diet, weight and health.